1. Field of the Invention
The subject invention relates to a conductive coating composition useful as a primer and/or as a conductive coating on metal surfaces of current collectors used in electrical energy storage devices which contain a non-aqueous electrolyte. The subject invention also relates to an additive for a conductive coating composition, a coated current collector, electrical energy storage devices containing a non-aqueous electrolyte incorporating any of the foregoing, and methods of making and using same.
2. Description of the Prior Art
Electrical energy storage devices containing non-aqueous electrolyte, such as lithium ion batteries and capacitors, generally include active coatings applied to a current collector to serve as a coated current collector, for example an electrode.
Lithium-ion batteries are mainly composed of a cathode and an anode, each of these electrodes comprising a different active material deposited on a current collector, in contact with electrolyte and separated by a separator. Generally, conductive metals are used as the current collector and there are several forms for current collectors: mesh, foam, foil, and the like. Metallic foils which are thin and light are preferred to improve volumetric capacity of cells. Current collectors possess high electrical conductivity to reduce cell resistance and desirably exhibit chemical stability in contact with electrolyte over the electrical potential operation window of the electrodes. Metal current collectors, while highly conductive, are often attacked by electrolyte which tends to shorten battery life.
Active coatings comprise an active material typically containing lithium, such as by way of non-limiting example, lithium iron phosphate and/or various lithium metal oxides, conductive additives and various binders. Cathodes are either oxides or phosphates containing lithium and first row transition metals, while anodes are commonly based on either carbon, such as graphite, or lithium titanium oxides.
Problems found in electrical energy storage devices containing non-aqueous electrolyte, in particular liquid electrolyte, include corrosion of the metal surface of the current collector by the electrolyte, as well as unsatisfactory adhesion of the active coating to the current collector. Both of these defects reduce efficiency and life of the electrical energy storage devices. One way to reduce such problems has been to apply a primer coating onto the metal surfaces of the current collector prior to application of the active coating. Although conductive primer coatings are commercially available, drawbacks of these products include poor electrical conductivity of the primer, i.e. lowered conductivity of the coated current collector; inadequate adhesion of the primer layer to the current collector and/or the active coating, as well as electrolyte attack on the primer coating and the underlying current collector. Another drawback of commercial conductive primers is the use of undesirable solvent-based carrier.
To improve electrical energy storage device, e.g. battery and capacitor, performance, there remains a significant need to reduce electrical resistance of current collectors having dried conductive coatings deposited thereon. As such a significant need exists to reduce electrical resistance of these dried coatings, including dried primer layers.
There remains a significant need to improve the resistance (e.g., insolubility and/or non-reactivity) of conductive coatings, in particular the conductive primer coatings, to non-aqueous electrolyte. Solving this problem is particularly challenging in the use environment, where the conductive primer coatings are exposed to electrolyte, e.g. liquid electrolyte, within electrical energy storage devices, under varying electrochemical conditions during discharge and recharge as well as at temperatures that vary from ambient to elevated temperatures. In addition, there is a need for conductive primer coatings that remain electrochemically stable over a broad range of cell voltages, that is a conductive primer coating that does not swell, delaminate, dissolve and/or react with electrolyte is still needed. This improved resistance to electrolyte desirably should not be made at the expense of conductivity and adhesion of the dried layers. There also remains a significant need to improve the adhesion properties of conductive coatings, in particular the conductive primer coatings, to the current collector. Also desirable is an active coating, having improved adhesion to the current collector, which can be applied directly to the current collector in the absence of a primer.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred; however the scope of the invention includes equivalents which are outside these limits. Also, throughout the description, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight or mass; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer” and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word “mole” means “gram mole”, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules. For purposes of water-solubility of polymers, the solubility is that determined at 1 atmosphere of pressure at 25° C. Generally, water-soluble polymers are those that will form a homogeneous solution with water, while a water-insoluble polymer will remain as a separate organic phase, which is detectable by methods known in the art. The terms “solution”, “soluble”, “homogeneous”, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25° C.
A “conductive coating composition” as used herein includes compositions that form coatings, which conduct electricity when the compositions are dried and/or cured coatings.
The term “(meth)acryl-” will be understood by those of skill in the art to refer to methacryl- and acryl-materials, e.g. (meth)acrylate refers to both acrylate and methacrylate materials.